Tattersall Glenn J, Ultsch Gordon R
Department of Biological Sciences, Brock University, St. Catharines, Ontario, L2S 3A1, Canada.
Biol Rev Camb Philos Soc. 2008 May;83(2):119-40. doi: 10.1111/j.1469-185X.2008.00035.x.
In cold-temperate climates, overwintering aquatic ranid frogs must survive prolonged periods of low temperature, often accompanied by low levels of dissolved oxygen. They must do so with the energy stores acquired prior to the onset of winter. Overwintering mortality is a significant factor in their life history, occasionally reaching 100% due to freezing and/or anoxia. Many species of northern ranid frogs overwinter in the tadpole stage, which increases survival during hypoxic episodes relative to adults, as well as allowing for larger sizes at metamorphosis. At temperatures below 5 degrees C, submerged ranid frogs are capable of acquiring adequate oxygen via cutaneous gas exchange over a wide range of ambient oxygen partial pressures (PO(2)), and possess numerous physiological and behavioural mechanisms that allow them to maintain normal rates of oxygen uptake across the skin at a relatively low PO(2). At levels of oxygen near and below the critical PO(2) that allows for aerobic metabolism, frogs must adopt biochemical mechanisms that act to minimise oxygen utilisation and assist in maintaining an aerobic state to survive overwintering. These mechanisms include alterations in mitochondrial metabolism and affinity, changes in membrane permeability, alterations in water balance, and reduction in cellular electrochemical gradients, all of which lead to an overall reduction in whole-animal metabolism. Winter energetic requirements are fueled by the energy stores in liver, muscle, and fat depots, which are likely to be sufficient when the water is cold and well oxygenated. However, under hypoxic conditions fat stores cannot be utilised efficiently and glycogen stores are used up rapidly due to recruitment of anaerobiosis. Since ranid frogs have minimal tolerance to anoxia, it is untenable to suggest that they spend a significant portion of the winter buried in anoxic mud, but instead utilise a suite of behavioural and physiological mechanisms geared to optimal survival in cold, hypoxic conditions.
在寒温带气候中，越冬的水栖蛙科蛙类必须度过漫长的低温期，且往往伴随着低水平的溶解氧。它们必须依靠冬季开始前积累的能量储备来做到这一点。越冬死亡率是它们生活史中的一个重要因素，偶尔会因结冰和/或缺氧而达到100%。许多北方蛙科蛙类以蝌蚪阶段越冬，这相对于成年蛙在缺氧期能提高存活率，同时也能在变态时达到更大的体型。在5摄氏度以下的温度时，潜入水中的蛙科蛙类能够在很宽的环境氧分压（PO₂）范围内通过皮肤气体交换获取足够的氧气，并且拥有众多生理和行为机制，使它们能够在相对较低的PO₂下维持正常的皮肤吸氧率。在接近和低于允许有氧代谢的临界PO₂的氧气水平时，青蛙必须采用生化机制来尽量减少氧气利用，并协助维持有氧状态以度过越冬期。这些机制包括线粒体代谢和亲和力的改变、膜通透性的变化、水平衡的改变以及细胞电化学梯度的降低，所有这些都会导致整个动物代谢的总体降低。冬季的能量需求由肝脏、肌肉和脂肪库中的能量储备提供，当水温较低且含氧量充足时，这些储备可能是足够的。然而，在缺氧条件下，脂肪储备无法有效利用，由于无氧代谢的启动，糖原储备会迅速耗尽。由于蛙科蛙类对缺氧的耐受性极低，认为它们在冬季大部分时间埋在缺氧的泥浆中是站不住脚的，相反，它们会利用一系列行为和生理机制来在寒冷、缺氧的条件下实现最佳生存。
